1. Field of the Invention
The present invention relates to a hydraulic control system for an automatic transmission and, more particularly, to a control system of a hydraulic servo for applying/releasing the frictional engagement elements of a transmission mechanism of an automatic transmission.
2. Related Art
In the automatic transmission of the prior art, the hydraulic servo for applying/releasing the frictional engagement elements (e.g., brakes or clutches) arranged in the gear train so as to effect a shift is controlled by the hydraulic control system. In one mode, the feed pressure to the hydraulic servo is directly controlled by a pressure regulator valve which is fed on one hand with a solenoid valve as a control signal pressure according to the running state of the vehicle and on the other hand with the feed pressure as a feedback pressure.
In the control of the feed pressure to the hydraulic servo, generally speaking, at a shifting process, a relatively low oil pressure is fed to the hydraulic servo in a manner to correspond to a torque capacity required by the frictional engagement elements. At the end of the shift, a high oil pressure corresponding to the line pressure has to be finally fed to the hydraulic servo thereby to ensure the state in which the torque capacity of the frictional engagement element is sufficiently retained. In view of the aforementioned construction of the prior art, this construction has to control the feed pressure to the hydraulic servo, from a low level for achieving the engagement to a level as high as the line pressure after the engagement is achieved, wholly by the solenoid pressure. As a result, the feed pressure cannot be raised to the high pressure corresponding to the line pressure unless a high control gain is taken. With this high control gain, on the contrary, the change of the feed pressure in relation to the change of the solenoid pressure excessively increases at the time of controlling the feed pressure at a low level before achievement of the engagement so that the feed pressure is highly dispersed with a slight dispersion of the solenoid valve thereby to deteriorate the pressure regulating accuracy.
As disclosed in Japanese Patent Laid-Open No. 215220/1993, there has been a technique in which the control gain is set to a low value, while ensuring the rise of the oil pressure to the line pressure, to improve the pressure regulating accuracy by controlling the pressure regulation using the solenoid pressure, while the level of the feed pressure to the hydraulic servo is being raised during a shifting process, with the use of the solenoid pressure and by interrupting the drainage of the feed pressure to establish a non-control state of no drainage when the level of the feed pressure is raised after the end of the shift to a level as high as the line pressure. Specifically, a pressure regulator valve to be controlled by the solenoid valve is disposed in a feed oil passage to the hydraulic servo of the frictional engagement element, and a change-over valve is disposed in the drain oil passage of the pressure regulator valve, so that the drainage after the control of the feed pressure of the pressure regulator valve by the solenoid pressure is interrupted by the change-over valve. However, this construction raises a problem that the pressure regulating accuracy is deteriorated.